1. Field of the Invention
This invention relates to a process for oxidizing cyclohexane with a gas containing molecular oxygen whereby adipic acid precursors are prepared in a manner which permits improved yield of adipic acid. More specifically, this invention relates to a process for oxidizing cyclohexane with a gas containing molecular oxygen, such as air, in the presence of a cyclohexane soluble cobalt catalyst and a phosphate ester to enhance the production of cyclohexyl hydroperoxide.
2. Description of the Prior Art
Adipic acid, among its other uses, is a well known intermediate in the manufacture of nylon, typically being used to react with hexamethylene diamine to form hexamethylene diammonium adipate which is polymerized by removal of water. Historically, adpic acid is usually prepared by the nitric acid oxidation of a mixture of cyclohexanone and cyclohexanol obtained from the oxidation of cyclohexane with a gas containing molecular oxygen, usually air, and optionally in the presence of a catalyst such as a cyclohexane-soluble cobalt compound. During this latter reaction, which is generally operated at low conversions in order to maximize the yield of cyclohexanone (K) and cyclohexanol (A), there is also produced cyclohexyl hydroperoxide (CHHP) which, depending on the oxidation conditions, is converted to K and A during the oxidation or in a separate step is decomposed to or converted to K and A.
One difficulty encountered in the oxidation of cyclohexane, particularly the catalytic oxidation, is that the CHHP is not only readily decomposed to K and A, but also to undesirable by products such as CO, butane, monobasic acids, aldehydes, and the like.
In view of the above there is an incentive to maximize the CHHP produced in the oxidation of cyclohexane by avoiding its decomposition to K and A in that step and the attendant undesirable by product formation, and to decompose or convert the CHHP to K and A in a separate step under nonoxidizing, and less severe thermal conditions than those encountered in cyclohexane oxidation.
Processes for the production of CHHP during the oxidation of cyclohexane have previously been suggested.
Cates, et al., U.S. Pat. No. 2,851,496, disclose a process in which cyclohexane is oxidized with molecular oxygen, optionally in the presence of up to 5000 ppm of a cobalt-containing or chromium-containing catalyst, to provide a mixture containing K, A and CHHP. The resulting CHHP is subsequently decomposed to K and A by heating in the presence of a decomposition catalyst.
Pugi, U.S. Pat. No. 3,530,185, discloses a staged process for oxidizing cyclohexane, with or without catalyst, in which a mixture of gases containing oxygen is introduced into a stream of cyclohexane to give primarily K and A with lesser amounts of CHHP. In general, if catalyst is present the CHHP tends to be decomposed and the main products of the oxidation are K and A.
Rapoport, et al., U.S. Pat. No. 3,957,876, disclose a process for oxidizing cyclohexane to preferentially produce CHHP in which a cyclohexane-soluble cobalt salt, such as those contemplated in this invention, is used as catalyst, and the reaction is carried out in a series of zones. CHHP produced, as measured by the weight ratio of CHHP/(CHHP+K+A), is greater than 0.15.
Barnette, et al., U.S. Pat. No. 3,987,100 disclose an improvement in the Pugi and Rapoport et al. processes wherein cyclohexane is oxidized in the presence of a cyclohexane-soluble binary catalyst system comprising chromium and cobalt salts. The CHHP formed during the reaction is decomposed to K and A in the presence of the binary catalyst.
None of these known processes deal with the possibility of using a phosphorus compound during the oxidation step. However, others have suggested the use of phosphate salts prior to or during the oxidation of cyclohexane.
Bonnart, et al., U.S. Pat. No. 3,510,526, disclose a process for obtaining a high proportion of CHHP by the air oxidation of cyclohexane in the absence of catalyst in apparatus previously rendered passive by treatment with sodium pyrophosphate.
Kuessner, et al., U.S. Pat. No. 3,917,708, disclose a process for producing high yields of K and A by oxidizing cycloalkane in the presence of heavy metal salt oxidation catalyst, such as cobalt monoalkylphosphate or cobalt dialkylphosphate. Production of CHHP is not mentioned.
Sipos, European Patent Application No. 0063931, discloses a process for the oxidation of cycloparaffins, such as cyclohexane, using a binary catalyst system of a cobalt compound and a chromium compound. The cobalt compound may be a dialkylphosphate, and in particular cobalt bis[di(2-ethylhexyl)phosphate]. Free dialkyl phosphate, e.g., di(2-ethylhexyl)phosphate, may be present in the oxidation catalyst. However, enhanced CHHP production is not achieved with the binary catalyst system.
Zelonka, U.S. Pat. No. 4,341,907 discloses a process for oxidation of cycloparaffins, such as cyclohexane, using a catalyst system comprising a cobalt compound in combination with a heterocyclic nitrogen compound, such as pyridine. The cobalt compound is preferably cobalt bis[di(2-ethylhexyl)phosphate]. Significant enhanced CHHP production is not obtained with the phosphate ligand present in the stoichiometric quantity suggested.